1. Field of the Invention
The present invention relates generally to the field of wireless receivers and particularly to a method and apparatus for detecting carrier frequency and timing offsets to enhance the receiver performance under the effects of multi-path channel.
2. Description of the Prior Art
As computers have gained enormous popularity in recent decades, so have networking the same allowing for access of files by one computer from another. More recently and with the advent of wireless communication, remote and wireless networking of computers is gaining more and more notoriety among personal users, small and large business owners, factory facilities and the like.
With regard to the wireless networking of personal computers including laptops, a particular modem, namely modems adapted to the IEEE 802.11a or 802.11g industry standard, are commonly employed. That is, an antenna is placed inside or nearby the personal computer and an RF chip receives signal or data through the antenna and an analog-to-digital converter, typically located within the personal computer (PC), converts the received signal to baseband range. Thereafter, a baseband processor is employed to process and decode the received signal to the point of extracting raw data, which may be files transferred remotely and wireless, from another PC or similar equipment with the use of a transmitter within the transmitting PC.
There are several prior art apparatus and techniques for implementing 802.11a/g modem receivers, however, such prior art have not successfully utilized the fullest potential of the 802.11a/g modem. For example, the maximum rate of this type of modem device is 54 Mbits/sec. but in the presence of multi-path channel, use of current prior art methods and apparatus does not allow for reception of data at such rates. In fact, successful reception of data under multi-path channel conditions currently takes place at lower rates or may fail altogether.
Furthermore, in areas other than open locations, such as smaller office cubicles located within the inter structure of a building, prior art receivers are known to only operate at rates lower than that of the maximum operational rate of the modem, such as 54 Mbits/sec., thus preventing a user from receiving files from another PC at optimal rates. This is generally due to the prior art techniques' limitations in operating within multi-path channel conditions, which occur mostly in closed areas, such as those mentioned hereinabove. In particular, multi-path channel may degrade a receiver's ability to acquire and track carrier frequency and timing rate offsets. For 802.11a/g modems that modulate data using orthogonal frequency division multiplexing (OFDM), multi-path channel may lead to dramatic loss in data detection performance because such frequency offsets may result in constellation distortion (e.g. unwanted rotations) and destroy orthogonality between the carriers causing inter-carrier interference (ICI). Thus, for high performance 802.11a/g modems require precise carrier and timing offset controls.
In light of the foregoing, it is desirable to develop an OFDM modem that can receive signals, such as those specified by the 802.11a/g standards, accurately in the presence of multi-path channel and with the ability to achieve maximum data rate throughputs. The presence of multi-path channel should be mitigated in such a way as to reduce the effect of noise, constellation distortions and ICI caused by poor carrier and timing offset controls. Furthermore, it is essential that enhancing the performance of the OFDM receiver be accomplished without adding considerable complexity to the hardware resources employed in processing the received OFDM signal.